I need the four front seat supports for my 2A (the outer RH and LH, and the two "double" inners). I've found some second hand described as for a lightweight but they look exactly like what I need. Does anyone know if I'd get a nasty surprise when I offered them up? Are lightweight seat bases essentially the same dimensions as other Series? These things are quite hard to find it seems, let alone a decent set all at once.

Not really hard to find. A quick ebay search shows at least 6 sets and a few more individual ones. Depends if you are comfortable with the prices